1. The Field of The Invention
The present invention relates to an apparatus and process for measuring certain parameters of a sheet of material.
2. State of the Art
Systems are well known for measuring certain parameters of various sheet materials. For example, U.S. Pat. No. 3,757,122 teaches a system for measuring the basis weight of a moving sheet of paper. According to the patent, a gauging head is disposed with one part of the head on either side of the moving sheet of paper and the hed is mounted n a frame and adapted to travel in frame transversely of the sheet of paper. A radiation source is located in the lower head and a radiation detector is located in the upper head to receive radiation from the source. The amount of radiation received by the sensor is related to the basis weight of the paper, and thus the gauging system monitors the basis weight as the paper travels between the two heads.
Parameters other than the basis weight of paper can be measured by systems similar to the one described in U.S. Pat. No. 3,757,122. For example, U.S. Pat. No. 3,793,524 teaches a system for measuring the moisture content of a sheet of material such as paper. The system includes a source of infrared radiation located in a gauging head member disposed on one side of the sheet of material and a detector to receiver the radiation located in a head member disposed on the other side of the material. Other parameters of a sheet material such as opacity and thickness or caliper can also be measured by similar gauging systems.
In gauging systems of the type discussed above, we have found that the distance between the two gauging head parts can sometimes be significant in determination of the value of the parameter being measured. For example, in a system of the type discussed above to measure the basis weight of paper, we have found that measurement of the basis weight can be affected by the distance between the radiation source and the radiation detector even when the actual basis weight of the paper remains constant. We have also found that in practice, the distance between the two head members can sometimes vary in an unpredictable manner as they travel across the sheet of paper. Thus, the measured value of the basis weight can sometimes deviate from the actual value.